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Organic Chemistry
Lab Techniques
| Question | Answer | Answer pt.2 |
|---|---|---|
| What are the different separations that are performed in the lab? | 1) Liquid to solid: filtration or centrifugation coupled w/decantation 2) Liquid to solute: distillation, precipitation coupled w/ filtration, extraction, chromatography, or ion exchange 3) Liquid to liquid: distillation | 4) Solid to solid: recrystallization, density gradient columns, molecular sieves, acid-base extraction, sublimation, or column chromatography |
| What is the most common method for purifying a solid? | -Recrystallization -b/c the crystalline forms a solid that is the most pure | |
| The purification of a liquid is the most commonly carried out by which separation technique? | Distillation | |
| What separation technique is used for the purification of hygroscopic organic liquids? | -Distillation from a drying agent such as magnesium sulfate -The drying agent is used to remove water from the organic solvent | |
| What does a distillation do? | It removes a liquid from either another liquid or from a solute by exploiting their boiling point differences (Lower BP boils out first) | |
| What happens to a distillation once it's been heated? | The most volatile component converts to a gas more readily than the less volatile components | |
| How does distillation work? | -Vapor escapes surface of liquid & travels up column -Vapor collides w/inner walls of column where it condenses -Some condensed liquid drips into flask & some re-evaporates | -By the time it reaches the top of the column, it has gone through enough purification cycles that nearly 100% of vapor is the more volatile component |
| What are the differences between simple and fractional distillation? | -FD has more surface area than SD -FD is used w/liquids w/small differences in B.P. (< 30 °C) while SD is used w/liquids w/large differences in B.P. -SD can also be used to remove a solvent from a solute | -FD has a ↑ distillate purity while SD is faster & generates a higher yield -FD is filled w/an inert solid to provide more surface area |
| What is vacuum distillation & what is its purpose? | -It involves attaching a vacuum to a distillation apparatus at a point after the condensate has been collected -Its purposes is to reduce the pressure in the apparatus & in doing so lower the boiling point | |
| Define boiling point | It is the temp @ which the vapor pressure equals the atmospheric pressure | |
| What kind of compounds are used in vacuum distillation? | Compounds that have really high boiling points | |
| What is chromatography? | It is the separation of two or more components in a mixture by exploiting their difference in solubility in a migrating solvent & their affinity for a polymer | |
| Define mobile & stationary phase | -Mobile phase (solvent/carrier): solvent moving through the column -Stationary phase (adsorbent): substance that stays fixed inside the column | |
| What materials are better for the stationary phase? | Polar materials as they have a higher affinity for the stationary phase than nonpolar materials EX: alumina or silica gel | |
| What happens when there is a polar species & nonpolar solvent? | It's slow migrating | |
| What happens when there is a nonpolar species & nonpolar solvent? | It's fast migrating | |
| What is thin layer chromatography (TLC) ? | It is carried out on a small scale to identify the # of components or type of components in a mixture | |
| How does TLC work? | -There is a rectangular plate w/either silica gel or alumina gel on the surface -A vertical line is made about 1 inch from bottom & small dot of sample is placed on the middle of line | -The plate is put into a container w/a lid & solvent is added below sample dot -Due to capillary action, the solvent slowly migrates up the plate & interacts w/each spot -Once solvent almost reaches the top it is removed from jar |
| How do you read the results from TLC? | It must be dried & using a UV, light you can see where the stops ended up | |
| TLC can't do what? | -It can't distinguish the relative abundance of enantiomers -Enantiomers travel at the same rate, so they cannot be separated | |
| Explain Rf vaules | -It can be thought as a "ratio of fronts", where the distance a spot travels is compared to the distance that the solvent travels - Rf values range = 0 ≤ Rf ≤ 1 | -The components w/the > Rf value is the component that has the least affinity for the stationary phase & is most soluble in that solvent |
| What is column chromatography? | -It is used to separate bulk quantities of product -Similar to TLC the solubility of a compound in the solvent as it flows down the column vs. the affinity of the compound for the adsorbent in the column | -For CC the "race to the finish line" is the bottom of the column & the components in the mixture separate according to how quickly they finish |
| In column chromatography what is the relationship between Rf value & elution time? | ↑ Rf value = ↓ elution time | |
| Define elution time | -It is the time it takes a component to travel the length of the column & drip out -Faster migration compounds reach the bottom first, meaning it has a shorter elution time | |
| Define stereogenic carbon | It is a carbon atom in a molecule that is attached to four different substituents making it a chairal center | |
| What is the difference between column chromatography in biochemistry & organic chemistry? | -Ochem = aim to have all compounds elute @ different times -Biochem = aim to have all but one compound elute | |
| What are the most common absorbents used in ion exchange chromatography on the MCAT and what are their charges at neutral pH? | -Sulfonated- polystyrene (-) -Carboxymethyl- cellulose ( CM-cellulose) (-) -Diethylaminiethoxy-cellulose (DEAE-cellulose) (+) | |
| What kind of proteins & amino acids will bind to CM-cellulose columns allowing them to be isolated? | -Proteins w/ high pI values -Ones rich in lysine, arginine, or histidine | |
| What kind of proteins & amino acids will bind to DEAE-cellulose columns allowing them to be isolated? | -Proteins w/ low pI values -Ones rich in aspartic acid or glutamic acid | |
| How does a sulfonated-polystyrene column work? | A protein mixture is added @ low pH so all the amino acids & small peptides residues bond to the columns. The pH is gradually ↑, releasing amino acids according to ↑ pI values | |
| How does gas chromatography work? | -It vaporizes a sample into a gas phase & it forces vapor through packed coils -The machine measures retention time on the column | |
| In gas chromatography what does a greater area for a signal indicate? | It indicates a greater quantity of material present | |
| In regards to silica gel or wax, what is a must for a compound to make sure a gas chromatography analysis works? | The compound sample must be highly volatile at 200°C & inert | |
| How is GC (gas chromatography) graph read and how can you tell the elution time? | -The graph is read from right to left -The signals on the right represents components w/ smaller elution time than components on the left | |
| How can you determine the purity of a product mixture in GC? | The more peaks present on the graph the purer it is | |
| What factors can dictate the migration rate of a compound in GC? | -Since most columns are slightly polar the affinity for polar species > nonpolar species -Heavier gases move slower & have longer elution times -Polar species + heavy gases = ↑ B.P. -Lower B.P. come out GC first | |
| Define extraction | It works based on solubility & it is typically employed to take advantage of drastic differences in the solubilities of components in two different (immiscible) solvents | |
| Define partitioning | It is when a compound has a different solubility in every solvent | |
| Define partition coefficinet | It is the ratio of its maximum solubility in one solvent compared to its maximum solubility in another solvent | |
| What is an acid/base extraction? | It involves partitioning between an organic solvent & water, where the pH is varied to enhance the solubility of compounds that can form ions when protonated or deprotonated | |
| What kind of water-sensitive compounds cannot undergo acid/base extraction? | Acid anhydrides, acid halides, and esters | |
| Define recrystallization | It involves dissolving the solid into hot solvent, hot filtering out the insoluble solid impurities, and then slowly cooling the solution to precipitate the purified crystals | |
| When picking the right solvent for recrystallization, what should the solvent be? | In order to maximize the amount of crystal collected, the desired material should be insoluble (or minimally soluble) in the solvent at lower temps | |
| What is decolorizing? | It is adding charcoal to solution to bind colored impurities | |
| Define sieve filtering | It is when the first filtration should be of a larger pore size than the second filtration so that the larger solid may be collected first & thereby separated from the smaller solid, which will then be collected in the second filter | |
| What is cold filtering? | The crystals at this point still have a residue of solvent w/soluble impurities on their surface. A cold solvent rinse is used to prevent the loss of product to dissolving | |
| What are the steps of the purification technique of recrystallization? | 1) picking a solvent 2) decolorizing 3) hot filtering 4) crystallization & cold wash | |
| What are the different identification techniques? | -Physical properties: melting & boiling points = common test (quick way to do it) -Chemical test: involves reagents that react w/a minimal # of functional group (color or phase change = + test & test agent must be limiting reagent in reaction) | -Derivatives: formed to provide more evidence as to identity of a compound (melting point is most common physical property, measured for a derivative) |
| What is the advantage of vacuum distillation over simple distillation? | Vacuum distillation facilitates the separation of compounds with extremely high boiling points | |
| What is UV-Vis spectroscopy? | It is used to obtain the absorbance spectra of a compound in solution or as a solid. It is mainly used to analyze conjugated systems |
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Gabbgabb04
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